1. Field of the Invention
The present invention relates to a field device, such as pressure and differential pressure transmitters, flowmeters, thermometers, and valve positioners, that are arranged in a plant in a decentralized manner. More specifically, the invention relates to the improvement of a field device and a signal transfer method for wirelessly transmitting additional information, including diagnostic information held by the field device, to a system that controls the field device.
2. Description of the Prior Art
In general, field devices are arranged at the location of petrochemical, food, pharmaceutical, water supply/sewerage, iron-making, or other process piping; where they detect physical quantities, including temperature, pressure and flow rate inside the piping; and transmit the detected signals to control equipment installed at a distant location or receive signals transmitted from the control equipment. Such field devices include pressure and differential pressure transmitters, flowmeters, thermometers and valve positioners.
There are prior art documents related to these field devices (see patent documents 1 to 3, for example, mentioned later in this section).
FIG. 11 is a conceptual block diagram of a valve positioner (hereinafter simply referred to as ‘positioner’) that is conceived according to the descriptions provided in the aforementioned patent documents. Positioner 10 receives a 4-20 mA current signal at signal input block 11 and measures the position sensor signal Position received by sensor signal input block 13 at signal processing circuit 12, wherein signal processing circuit 12 outputs a manipulation signal to valve actuating block 14, so that valve 20 is placed in a position corresponding to the received 4-20 mA current signal.
Most of the positioners known as smart-type positioners contain a pressure sensor (not shown in the figure) for measuring the manipulation pressure applied to valve 20. The pressure signal Pressure of the pressure sensor is input to sensor signal input block 13 and is processed appropriately by signal processing circuit 12 so that the manipulation pressure can be monitored. In addition to monitoring the manipulation pressure, the positioner makes it possible to diagnose valve 20 from various aspects, including the valve position.
The position sensor and pressure sensor signals or the results of valve diagnosis are transmitted from smart signal transmitter/receiver block 15 to a host system by means of so-called smart communication. Smart communication refers to digital communication in which digital signals are superimposed on 4-20 mA analog signals, and is also called hybrid communication. Methods of smart communication in practical use include HART, Brain and DE.
The host system may be a handheld communicator (HHC) or an asset management system.
As field devices have become increasingly intelligent recently, there has been a growing demand to optimize equipment maintenance costs by transmitting a variety of process operation information and diagnostic information to an asset management system.
However, existing distributed control systems (DCS) have the following problems:
In some cases, it is difficult to connect the latest asset management system to an existing DCS. Under normal conditions, a multiplexer is added in order to connect a digital communication line from the asset management system to each individual positioner. Communication may not be possible, however, due to low impedance in the communication band of an I/O card's 4-20 mA loop.
In this case, a filter is inserted in series with the 4-20 mA loop in order to increase the impedance in the communication band.
In spite of this measure, the voltage that can be output on the DCS side may be exceeded for reasons of voltage drops.
Wireless transmission is a method of digital signal transfer free from such problems. A number of techniques for adding a wireless module to a field device to make this transfer feasible have already been proposed.
By adding a wireless module to a field device instrumented using the existing 4-20 mA signal interface, it is possible to transmit a variety of process operation information and diagnostic information on field devices to the asset management system. FIG. 12 is a block diagram illustrating one example of a positioner configured in the manner described above, wherein RF unit 16 is a wireless module.
Patent document 1: Japanese Laid-open Patent Application 2003-134261
Patent document 2: Japanese Laid-open Patent Application 2003-051894
Patent document 3: Japanese Laid-open Patent Application 2004-021877
Patent document 4: Japanese National Publication of Patent Application 1998-508129
Patent document 5: Japanese Patent 3635982